g3 sponges. phylum porifera 



porifera: general features 



The sponges are known in zoology as Porifera. In that their 

 bodies consist of many ' cells ', they might seem to be Metazoa. 

 But thev differ from all members of that group in several 

 important respects. In no metazoan are choanocytes found. In 

 none is the principal opening exhalant. In none is there during 

 development an inversion whereby a flagellated outer covering 

 becomes internal. Lastly, and perhaps most significantly, in a 

 sponge the ' cells ' are far less speciaHsed and dependent upon 

 one another than the cells of a metazoan. Many of them can 

 assume various forms, becoming amoeboid, collared, etc. Many 

 are isolated in the jelly, and when they touch there is no cyto- 

 plasmic continuity. There is no nervous system. Even the 

 choanocytes, though their efforts together produce a current, do 

 not keep time in their working. In short, the Porifera are practi- 

 cally colonies of Protozoa. For these reasons it is best that, in 

 a classification of animals, they should be given the same rank as 

 the Protozoa and Metazoa : when this is done, they are known as 

 phylum Porifera, sub-kingdom Parazoa. 



